Recording how cells in the brain of an organism pass signals to each other can help researchers understand the neural underpinnings of behavior. But it has remained challenging to quickly image such neural activity over large volumes of tissue inside the live brain.

In a study published Sept. 7 in the journal Biomedical Optics Express, Martin Booth and colleagues at the University of Oxford introduced a microscopy approach that is optimized for rapidly monitoring neural signals over continuous volumes in the intact fruit fly brain. The technique, called adaptive optics-based volumetric activity sensing two-photon (AO-VAST) microscopy, achieves high-speed volume imaging through the reduction of spatial resolution.

The resolution of a standard two-photon microscope (about 0.5 μm) is smaller than some neurons. For example, the cell bodies of specialized neurons called Kenyon cells, which are found in fruit fly and some other invertebrates’ brains, are up to 5 μm wide. So, the researchers could reduce their microscope’s spatial resolution while still achieving close to single-cell resolution.

The resulting optical sections are thicker than the sections that can be captured with standard two-photon microscopes, so fewer images and less time are needed to cover the whole volume. The optical system itself is capable of imaging 100 volumes per second.

The researchers used AO-VAST microscopy to record how Kenyon cells in fruit flies that were awake responded to odors. They imaged a volume of 50 × 80 × 20 μm, consisting of a set of 5 slices (each about 5 μm thick) separated by 5 μm. The fast sensing method reproducibly captured the responses of cell groups that responded to different odor stimuli. Moreover, the approach avoids the loss of data due to the motion of live specimens. According to the authors, the compact footprint of the system means it could be easily added to commercial two-photon microscopes when researchers want to quickly image large volumes.